Generally, a printing using a color hard copy printing paper is carried out, in which a dye carrier paper coated with an ink made of a sublimation dye by heating with a thermal print head to sublimate the dye formed on the necessary portion, whereby a color picture image is formed on the printing paper.
However, the picture image formed by this sublimation transfer method has the following problems.
Although at first the sublimated dye is adsorbed by the surface of the printing paper, it is not diffused well so that a portion of the dye may remain as an aggregate, and hence the sublimated dye can not be developed as intended.
Further, the dye is again aggregated, crystallized, and discolored or the dye is transferred to another material and thereby it is smudged
To solve these problems, there are proposed in the art the following methods but these methods have their own defects.
(a) As to the method involving coading the printing paper with a resinous liquid, the picture image is blurred. Thus, the printing paper must be dried and the printing paper may not provide a smooth surface. Further, the resinous liquid is troublesome to handle.
(b) When a method of laminating a polyethylene terephthalate film having a hot melt adhesive agent layer on the printing paper is employed, the film becomes easy to curl, to absorb dusts, or to be bent or the adhesive agent is apt to spread excessively. Further, if set incorrectly upon laminating, the laminating apparatus is smudged or the film is caught in the laminating apparatus and thus the handling of this film is cumbersome.
(c) When a method of sandwiching the color hard copy between the polyethylene terephthalate films having a hot melt adhesive agent layer is employed, there may occur problems such as the appearance of the film is damaged and a bubble is produced in the film by the water component contained in the paper.
To solve these problems, there has been proposed a transfer type thin cover film for color hard copy printing paper.
However, since this cover film for color hard copy printing paper is thin, there occur troubles that when the printed color hard copy covered by the cover film is left in the inside of a car in summer, near a window in summer or is used under conditions of high temperature and high humidity, the dye is moved to the surface of the cover film and thereby an album used for preserving pictures and other color picture images is smudged. Although this phenomenon is not yet explained explicitely, it may be considered that a very small amount of dye dissolved or dispersed into the water component in the resin moves readily to the surface of the film together with the water component or that the glass transition temperature of the resin is lowered by the water component.
In view of the above mentioned aspects, the present invention is intended to provide a cover film for hard copy printing paper which can remove bad effects exerted by the transfer of the dye even under the condition of high temperature and high humidity.